ixgbe: Resolve NULL reference by setting {read, write}_reg_mdi

Set the read_reg_mdi and write_reg_mdi method pointers for
X550EM_A_10G_T devices to resolve jumping to NULL.

Signed-off-by: Mark Rustad <mark.d.rustad@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>

drivers/net/ethernet/intel/ixgbe/ixgbe_x550.c

@@ -3049,6 +3049,8 @@ static const struct ixgbe_phy_operations phy_ops_x550em_a = {
 	.identify		= &ixgbe_identify_phy_x550em,
 	.read_reg		= &ixgbe_read_phy_reg_x550a,
 	.write_reg		= &ixgbe_write_phy_reg_x550a,
+	.read_reg_mdi		= &ixgbe_read_phy_reg_mdi,
+	.write_reg_mdi		= &ixgbe_write_phy_reg_mdi,
 };
 
 static const u32 ixgbe_mvals_X550[IXGBE_MVALS_IDX_LIMIT] = {